When porcine somatotropin (PST) is administered to pigs on a daily basis a marked improvement in feed efficiency, i.e., feed-to-gain, the animal takes in less food than control animals while gaining weight at the same or greater level. However, it is inconvenient to administer drugs to the swine on a daily basis because of the large expense and amount of time required to deliver the drug to each member of a large group of animals. It would, therefore, be more feasible to apply a single dose and have the PST release over a prolonged period of time. It would appear the easiest way to administer PST would be in a feed, however, like most proteins and macromolecular drugs, PST is not orally bioavailable. In general, the literature has not shown significant ability to deliver proteins and peptides via the oral route. The use of salicylates and mineral oil to enhance oral delivery of these drugs is disclosed in E.P. Application No. 0177342. U.S. Pat. No. 4,548,922, further discloses the use of steroid enhancers, such as fusidic acid.
Parenteral administration, for example, implants have been employed with other macromolecular drugs to give a prolonged release as disclosed in U.S. Pat. No. 4,666,704. PST is an unstable protein susceptible to enzymatic, as well as aqueous degradation. It reacts with itself and is readily cleaved by proteases. Indeed, the instability of the PST implant is thought to be due to the proteases that are generated at the implantation site because of the inflammatory response that takes place (U.S. Pat. No. 5,015,627). It is because of this instability that the administration of PST to swine has hitherto been only marginally, if at all successful. (See for example: U.S. Pat. Nos. 4,837,381; 4,857,505; 5,045,312, and EP Applications 0193917 and 0458064)
There is a commercial need for improved prolonged release implants for parenteral administration of macromolecular drugs having growth hormone activity.
It is, therefore, an object of this invention to provide a composition and method for stabilizing and releasing a biological active growth hormone in animals over a prolonged period of time.
It is a further object of this invention to provide a stable form of PST which will release in swine over a prolonged period of time and which results in improved bioavailability and feed efficiency.